MC33365(2006) Просмотр технического описания (PDF) - ON Semiconductor
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MC33365 Datasheet PDF : 11 Pages
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MC33365
The Power Switch is designed to directly drive the converter
transformer and is capable of switching a maximum of
700 V and 1.0 A. Proper device voltage snubbing and
heatsinking are required for reliable operation.
A Leading Edge Blanking circuit was placed in the current
sensing signal path. This circuit prevents a premature reset
of the PWM Latch. The premature reset is generated each
time the Power Switch is driven into conduction. It appears
as a narrow voltage spike across the current sense resistor,
and is due to the MOSFET gate to source capacitance,
transformer interwinding capacitance, and output rectifier
recovery time. The Leading Edge Blanking circuit has a
dynamic behavior in that it masks the current signal until the
Power Switch turn−on transition is completed. The current
limit propagation delay time is typically 262 ns. This time is
measured from when an overcurrent appears at the Power
Switch drain, to the beginning of turn−off.
Error Amplifier
An fully compensated Error Amplifier with access to the
inverting input and output is provided for primary side
voltage sensing, Figure 17. It features a typical dc voltage
gain of 82 dB, and a unity gain bandwidth of 1.0 MHz with
78 degrees of phase margin, Figure 6. The noninverting
input is internally biased at 2.6 V ±3.1% and is not pinned
out. The Error Amplifier output is pinned out for external
loop compensation and as a means for directly driving the
PWM Comparator. The output was designed with a limited
sink current capability of 270 μA, allowing it to be easily
overridden with a pull−up resistor. This is desirable in
applications that require secondary side voltage sensing.
Bulk Capacitor Voltage Comparator
In order to avoid output voltage bouncing during
electricity brownout condition, a Bulk Capacitor Voltage
Comparator with programmable hysteresis is included in
this device. The non−inverting input, pin 11, is connected to
the voltage divider comprised of RUpper and RLower as
shown in Figure 20 monitoring the bulk capacitor voltage
level. The inverting input is connected to a threshold voltage
of 1.25 V internally. As bulk capacitor voltage drops below
the pre−programmed level, (Pin 11 drops below 1.25 V), a
reset signal will be generated via internal protection logic to
the PWM Latch so turning off the Power Switch
immediately. An internal current source controlled by the
state of the comparator provides a means to program the
voltage hysteresis. The following equation shows the
relationship between VBULK levels and the voltage divider
network resistors.
RUpper + 20 [ VBulk_H * VBulk_L ] in K Ohm
25
RLower +
[ VBulk_H * VBulk_L ]
VBulk_H * 1.25
in K Ohm
VBULK
Vref
RUpper
BOK
11
RLower
50 mA
1.25 V
Protection Logic
Figure 20. Bulk OK Functional Operation
Undervoltage Lockout
An Undervoltage Lockout comparator has been
incorporated to guarantee that the integrated circuit has
sufficient voltage to be fully functional before the output
stage is enabled. The UVLO comparator monitors the VCC
voltage at Pin 3 and when it exceeds 14.5 V, the reset signal
is removed from the PWM Latch allowing operation of the
Power Switch. To prevent erratic switching as the threshold
is crossed, 5.0 V of hysteresis is provided.
Startup Control
An internal Startup Control circuit with a high voltage
enhancement mode MOSFET is included within the
MC33365. This circuitry allows for increased converter
efficiency by eliminating the external startup resistor, and its
associated power dissipation, commonly used in most
off−line converters that utilize a UC3842 type of controller.
Rectified ac line voltage is applied to the Startup Input,
Pin 1. This causes the MOSFET to enhance and supply
internal bias as well as charge current to the VCC bypass
capacitor that connects from Pin 3 to ground. When VCC
reaches the UVLO upper threshold of 15.2 V, the IC
commences operation and the startup MOSFET is turned
off. Operating bias is now derived from the auxiliary
transformer winding, and all of the device power is
efficiently converted down from the rectified ac line.
The startup MOSFET will provide a steady current of
1.7 mA, Figure 11, as VCC increases or shorted to ground.
The startup MOSFET is rated at a maximum of 400 V with
VCC shorted to ground, and 500 V when charging a VCC
capacitor of 1000 μF or less.
Regulator
A low current 6.5 V regulated output is available for
biasing the Error Amplifier and any additional control
system circuitry. It is capable of up to 10 mA and has
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